“Infrasound” is conventionally regarded as sound at frequencies less than 20 Hz. Airborne infrasound is thus defined by pressure waves at frequencies below 20 Hz. The human ear cannot hear infrasound, but at higher amplitudes it can be felt by the body. These waves propagate with little attenuation over long distances, carrying information about the state of the atmosphere across the planet. At any given area, the change in the infrasonic field can be an indicator of, for instance, an approaching storm, a large amplitude explosion (such as those monitored for in accordance with the Comprehensive Test Ban Treaty), or an earthquake on the other side of the planet.
Infrasound is typically measured from the earth's surface with a noise-reducing line microphone. See, e.g., Bedard Jr., A. J., “Low-Frequency Atmospheric Acoustic Energy Associated with Vortices Produced by Thunderstorms,” Monthly Weather Review, Vol. 133, January 2015, hereby incorporated herein by reference. The sensor of Bedard operates by reducing wind noise through a spatially distributed measurement over a circular area of up to 50 meters in diameter. The spatial distribution of Bedard's sensor averages out incoherent noise while reinforcing the spatially coherent signals, such as propagating infrasonic waves. An infrasonic array of such sensors is formed with a spacing of one quarter of the primary wavelength of interest (about 40-80 meters) and yields the propagation direction as well as in-plane phase speed.
The primary drawback of Bedard's sensor is that it is stationary and on the earth's surface. The latter characteristic of Bedard's sensor contaminates the measurement of infrasonic atmospheric waves with near-surface waves excited by seismically-induced vertical motion of earth's surface. Another drawback of Bedard's sensor is that the pneumatic transmission line spatial averaging becomes ineffective beyond certain wind speeds.
A compact, extremely low frequency microphone has been developed at NASA. See Qamar A. Shams and Allan J. Zuckerwar, “Extreme Low Frequency Acoustic Measurement System,” U.S. Pat. No. 8,401,217 B2, issued 19 Mar. 2013, hereby incorporated herein by reference. See also, Allan J. Zuckerwar and Qamar A. Shams, “Sub-Surface Windscreen for Outdoor Measurement of Infrasound,” U.S. Pat. No. 8,671,763 B2, issued 18 Mar. 2014, hereby incorporated herein by reference; Qamar A. Shams and Allan J. Zuckerwar, “Extreme Low Frequency Acoustic Measurement System,” U.S. Pat. No. 9,591,417 B2, issued 7 Mar. 2017, hereby incorporated herein by reference; and Qamar A. Shams, Allan J. Zuckerwar, and Howard K. Knight, “Wake Vortex Avoidance System and Method,” U.S. Pat. No. 9,620,025 B2, issued 11 Apr. 2017, hereby incorporated herein by reference.
According to Q. A. Shams and A. J. Zuckerwar in their two related patents noted above, a specifically designed windscreen covers an electret microphone to filter out wind-noise. However, a drawback of the sensor of Shams and Zuckerwar is that it has a small spatial aperture; therefore, incoherent noise at scales larger than the sensor's aperture cannot be mitigated with a single sensor. The sensor of Shams and Zuckerwar is a ground-based sensor, and as such is typical of the current state of the art.
Interferometric-based fiber optic sensors have a low sensor noise floor in the near-infrasound (1-20 Hz). The interferometer senses an acoustic field through the pressure-induced changes to the fiber length, diameter, and/or index of refraction. Increasing the length of the fiber increases the sensor's sensitivity. Most importantly, the fiber-optic sensor averages out incoherent pressure fluctuations along the fiber of length L as
            p      _        ⁡          (      t      )        =            L              -        1              ⁢                  ∫        0        L            ⁢                        p          ⁡                      (                          l              ,              t                        )                          ⁢                                  ⁢                  dl          .                    This attribute makes a large aperture fiber optic sensor effective at averaging out incoherent noise such as wind and other sources of ambient turbulence.
An example of infrasound sensing viability is disclosed by M. A. Zumberge, “An optical fiber infrasound sensor: A new lower limit on atmospheric pressure noise between 1 and 10 Hz,” J. Acoust. Soc. Am., 115 (5), 2003, hereby incorporated herein by reference. As disclosed by Zumberge, a land-based optical fiber infrasound sensor has been shown to measure pressure down to 707 μPa Hz−1/2 (57 dB re μPa Hz−1/2) at 1 Hz. At 6 Hz the minimum noise floor for Zumberge's sensor drops to 31.6 μPa Hz−1/2 (30 dB re μPa Hz−1/2). The device of Zumberge is a Mach-Zender interferometer with two equal optical fiber arms.
According to a fabricated embodiment of Zumberge's device, the arms were wrapped around a sealed compliant 89 meter long tube with a diameter of 2.5 cm. The two fibers were configured in a way to minimize noise due to temperature, via common mode rejection, while maximizing the strain differential between them, which was directly proportional to the interferometric response. In order to further reduce wind noise, Zumberge's sensor was buried in gravel 20 cm below the surface. See also, Mark Zumberge et al., “Optical Fiber Infrasound Sensor,” U.S. Pat. No. 6,788,417, issued 7 Sep. 2004, hereby incorporated herein by reference.
Infrasound sensing can be extremely valuable for forecasting weather and for other important purposes such as detecting and monitoring severe weather (hurricanes, monsoons, tornadoes, tropical storms, clear air turbulence, Comprehensive Nuclear Test-Ban Treaty (CTBT) events such as large explosions, and natural and man-made seismic events (geologic vibrations, earthquakes, volcanoes, encounters with celestial and man-made objects such as meteors, asteroids, artificial satellites, spacecraft, rockets, missiles, airplanes, helicopters, etc. These objectives would be better served by better methods and systems for performing infrasonic sensing of the earth's atmosphere.